Yellowish white crystal aggregates of the rare Tantalum mineral rankamachite from the TL in Congo (Mumba Cassiterite Gravels, Mumba, Masisi Mountains, North Kivu, DR Congo). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Rankamaite
Rankamaite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula (Na,K)₃(Ta,Nb,Al)₁₁(O,OH)₃₁.
It crystallizes in the orthorhombic crystal system (space group C222).
Key properties: Mohs hardness 3–4; specific gravity 5.5; Silky luster; optically biaxial.
IMA symbol Rkm, IMA status: approved, named in 1968.
Apparently a replacement of simpsonite, the result of intense sodium metasomatism in some Li-Ta-Nb-bearing granite pegmatites.
It is found associated with Simpsonite, microlite, cassiterite, beryl, manganotantalite, muscovite, kaolinite (Mumba district, Congo); lithiotantite (Eastern Kazakhstan).
The mineral was named to honor Professor Kalervo Rankama (1913–1995), Finnish geochemist, University of Helsinki, Helsinki, Finland.